1. Field of the Invention
The present invention relates to a screw fastening machine of first screwing in screw components such as bolt and nut at high speed rotation, and then fastening the screw components to a work at a final fastening torque set in advance.
2. Description of the Related Art
A machine that automatically supplies a component at every work cycle, and fastens the component to a work is well known as an automatic screw fastening machine of bolt, nut, screw and the like that is generally well known in the conventional art. One example of such screw fastening machine has a configuration that uses a bit engageable to a drive hole having a rectangular shape such as cross-shape or hexagon socket formed at a head of the screw and connects the bit to an electrically operated driver by way of a bit drive shaft. When fastening the screw to the work with such screw fastening machine, a electrically operated driver is driven with the bit engaged to the drive hole of the screw. A rotation force is then transmitted from the bit to the screw, and the screw is fastened to the work.
Such screw fastening machine is not suited for screw fastening work in areas where high screw fastening precision (fastening torque with small variance) is demanded, and is not frequently adopted when the fastening torque must comply with the rule. The fastening machine shown in FIG. 4 is thus proposed as one example of the screw fastening machine that have been recently used as the screw fastening machine of stable fastening torque. This is the screw fastening machine disclosed in Japanese Laid-Open Patent Publication No. 60-052268, where a low speed high torque driver 130 and a high speed low torque driver 110 are simultaneously driven after the screw is held at a chuck unit (not shown). In this case, an outer ring 141 slips with respect to an inner ring 144 by a one-way clutch 140, and a driver bit (not shown) rotates in response to the rotation of only the high speed low torque driver 110 via a driver shaft 105 to fasten the screw at high speed. In this operation, the number of rotations of the high speed low torque driver 110 lowers as the screw gets fastened and when becoming lower than the number of rotations of the low speed high torque driver 130, the rotation of the low speed high torque driver 130 is transmitted to a gear coupled driver shaft 105 by the operation of the one-way clutch 140, thereby screw fastening to a final fastening torque at low speed.